Don't copy the context menu of ViewBoxes.

This allows customization of the context menu of a ViewBox simply by
calling viewbox.menu.addAction(...).  See issue #13.  Also some cleanup.
Override ViewBox.popup() to update menu before showing
Extend ViewBox menu in examples/contextMenu
Use actions of ViewBox's contextMenu in full menu.

The main change is on `ViewBox.getContextMenus`, which now returns an
up-to-date of actions that `GraphicsScene.addParentContextMenus` can use.

Also, `getContextMenus` was given a default implementation in the base
class (falling back on `getMenu` if defined), and some cleanup was done.
fixed context menu handling for non-GraphicsItems
Merge branch 'dont-copy-context-menu' into develop
This allows ViewBox context menus to be modified by directly manipulating ViewBox.menu